Hydraulic fluids



United States Patent Ofiice US. Cl. 252-78 16 Claims ABSTRACT OF THE DISCLOSURE Hydraulic fluid comprising a major portion of a trialkyl phosphate or mixed alkyl aryl phosphate and a minor portion of an antioxidant-anticorrodent combination of a compound having at least one unfused oxirane group, and an aliphatic polyhydroxy compound having 2 or more hydroxy groups, at least 2 of which are separated from earth other by no more than 3 carbon atoms.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 575,974, now abandoned, filed Aug. 30, 1966.

BACKGROUND OF THE INVENTION This invention relates to fluid compositions which are particularly useful for transmitting power in hydraulic systems. More particularly it relates to phosphorus ester fluids protected from oxidation and corrosion by a synergistic combination of inhibitors.

Various oleagenous materials have been employed in the past as fluids for use in hydraulic poweritransmission systems, A recent growing usehas been in the hydraulic systems of airplanes, wherein power is transmitted to operate controls, servos and various othermechanisms of the craft. Certain specific characteristics are demanded of a fluid thatis to be used in aircraft applications. The fluid must meet certain stringent functional requirements, and also it must possess the characteristic ofbeing relatively flame resistant in order to satisfy aircraft safety requirements. Additionally, the fluid should have a low pour point in order that it may function in low temperatures and a high viscosity index so that it may function over a wide temperature range. It also should have low volatility, at elevated temperatures. Other very important requirements are a high degree of'lubric'ity in order that pumps and motors, etc., will be sufliciently lubricated, and a low toxicity to facilitate handling. An additional requirement, of special interest in aircraft uses, is that the fluid have 'a relatively low density so as not to add excess weight to the craft. Among the most important materials which have been developed to meet the requirements of hydraulic systems in general and aircraft hydraulic systems in particular, are the numerousphosphate esters. These materials have been employed quite successfully in aircraft applica tions, ingeneral, meeting the requirements enumerated above better than almost any other material, especially when compounded with proper viscosity index improvers and other additives. Included among thevarious phosphate esters which have been employed are various triaryl esters, trialkyl esters, and mixed esters such as dialkyl monoaryl and diaryl monoalkyl esters. It has been the experience in the industry that when the hydraulic compositions are comprised primarily or triaryl materials such as tricresyl phosphate, or triphenyl phosphate, etc., that the high temperatures usually encountered have little effect upon the materials, neither oxidizing them nor causing them to beconie corrosive to metal surfaces. However, for the purposes of obtaining proper viscosities, it is usually necessary to employ a substantial proportion of either mixed alkyl aryl esters or trialkyl esters, for the viscosities of the triaryl materials are usually too high and the viscosity indices too low for proper operation. It has been the experience in the art, however, that the alkyl phosphates and mixed alkyl aryl phosphates, when employed at high temperatures, e.g., at temperatures above about C., are susceptible to oxidation and become extremely corrosive to copper, cadmium, magnesium, etc. Thus, it is usually advisable and often necessary to employ in the fluids certain antioxidants and anticorrodents.

SUMMARY In accordance with the present invention, it has now been found that fluids having fire-resistant characteristics and especially suitable for use in aircraft hydraulic systems can be compounded from a major portion of a trialkyl phosphate or mixed alkyl aryl phosphates or mixtures thereof and a minor portion of a synergistic mixture of inhibitors comprising an epoxy compound and an aliphatic polyhydroxy compound having 2 or more hydroxy groups, at least 2 of the hydroxy groups being separated from each other by no more than 3 carbon atoms.

DESCRIPTION The trialkyl phosphates of this invention may be represented by the following formula:

where X, Y, and Z are alkyl or substituted alkyl radicals,

straightor branched-chain, containing from 1 to 30, preferably 1 to 18, carbon atoms. Examples of suitable radicals include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tort-butyl, n-amyl, isoamyl, n-hexyl, 2-ethylbutyl, heptyl, octyl, Z-ethylhexynonyl, decyl, undecyl, dodecyl, hexadecyl, octadecyl, eicosyl, and tricontyl. Various combinations of the alkyl radicals may be present in the esters and various mixtures of the mixed alkyl esters may be used as fluids; e.g., a mixture of dibutyl monoethyl phosphate and amyl 'butyl propyl phosphates may be .used.

In the mixed alkyl aryl phosphates, one or both X and Y in the formula set forth above is an aryl radical. The

alkyl radicals will be the same as listed in the definition of the trialkyl materials. Thus, mixtures of alkyl radicals will be suitable. The aryl radicals include monoand polynuclear materials, substituted and unsubstituted. Thus, the aryl nuclei may be substituted by various alkyl groups, a-lkenyl, halo or haloalkyl, alkoxy alkenoxy, nitro, substituted and unsubstituted amino, alkylamino hydroxy, mercapto, etc. t

The aryl radicals thusinclude substituted and unsubstituted phenyl, naphthyl, anthranyl, phenanthryl groups, etc. The phenyl and substituted phenyl materials are preferred. Thus, examples of the preferred radicals include phenyl, xylyl, tolyl, etc.

The epoxy compounds which function as a part of the inhibitor combination include any compound containing by reference to 1,2-epoxydodecane, which is suitable, compared to epoxycyclododecane which is not suitable because of the fusion of theoxirane ring to the cycloalkane' ring.

Because many hydraulic systems are operated at elevated temperatures open to the atmosphere in some re- Patented Dec. 30, 1969,

have a sufliciently high boiling point that it will not be r The epoxy hydrocarbons will,.,contain at least 2 carbon atoms with an upper limit determined only by solubility and the ability to provide a suflicient oxirane content to the fluid. Thus, a monoepoxy tricontane will have an oxirane oxygen content of about 4% and this additive at 1% in the fluid will provide 0.04 oxirane content in the fluid. The epoxy hydrocarbons may be substituted with various groups and may contain aromatic and nonaromatic cyclic structures.

- However, it should be noted that as the introduction of sizable quantities of flammable materials into the phosphate ester fluids is preferably to be avoided, in order to keep the concentration low, materials should be used that will provide suflicient oxirane oxygen content at concentration levels of under about 5, and more preferably 3, percent.

The epoxy compounds will contain at least one nonfused oxirane ring. That is, while other cyclic structures may be present in the compound, the two carbon atoms of at least one oxirane group will not be in one of those structures. Thus, while epoxy vinyl cyclodecane is effective, epoxy cyclododecane is not. The presence of additional fused oxirane rings is acceptable; e.g., vinyl cyclohexene dioxide having one fused and one nonfused oxirane ring is an effective compound.

Included in the hydrocarbon epoxides are the epoxyalkanes such as ethylene oxide, propylene oxide, butylene oxide, epoxyhexane, epoxyoctane, epoxydecane, 1,2-epoxydodecane, 1,2-epoxyhexadecane, epoxyeicosane, epoxyheneicosane, epoxytricontane. Branched materials may be used. Included among these are epoxidized polyalkenes, such as propylene and butylene polymers, etc. As noted, the materials may include aromatic and nonaromatic nuclei; thus, compounds which have aromatic or nonaromatic nuclei joined to epoxidized alkenyl groups of various types may be used. Examples of such materials are 1,2- epoxy 3 phenylpropane, 1,2 epoxy 1,2 phenyldodecane, epoxyvinylcyclohexane, 3,4-vinyl-cyclohexenedioxide, etc. Residual unsaturation may also be present in the compounds.

The class of epoxy ethers which are especially effective are illustrated by the glycidyl ethers such as glycidyl phenyl ether, glycidyl allyl ether, various gylcidyl alkyl ethers including 1,2-epoxyoctoxypropane, 1,2-epoxyhexoxypropane, etc.

The very important class of effective epoxy fats are the epoxized unsaturated fats, especially the epoxidized drying oils. The unsaturated fats are triglycerides of various combinations of saturated and unsaturated acid, the most common acid-s being palmitoleic, oleic, ricinoleic, linoleic, linolenic, eleostearic, and licanic acids. Reference to the unsaturated acids may be found in Kirk-Othmer, Encyclopedia of Chemical Technology, volume 6, pp. 266-83: The drying oils are discussed in vol. 5, pp. 27799. The drying oils contain acids having a high degree of unsaturation and, in the case of marine drying oils, in addition to the unsaturated acids previously mentioned, containing large proportions of 2.0, 22 and 24 carbon acids with large numbers of double bonds. A common acid found in fish gils is clupanodonic acid, a 22 carbon acid having double onds.

Examples of the drying oils which yield effective h hi or s when epoxidized include linseed oil, tung oil,

soybean oil, fish oils, oiticica oil, etc. The oils may be modified in various ways in..addition to epoxidation. For example, oils containing ricinoleic acid may be acylated on the hydroxy radicals'to yield various esters of the triglycerides before epoxidationQ One such compound is a glyceryl tris-(acetoxy epoxy) stearate. 1 Y

In addition to the epoxidized fats and oils, other suitable compounds include epoxidized monoesters of unsaturated acids, for example, butyl epoxy stearate, isooctyl epoxy stearate, etc.

An example of a preferred epoxide is an epoxidized linseed oil. A commercially available material of this type s PQ QI 7-5 s d y. Swift nd C0 having a m m m oxirane oxygen content of 9% by weight. 7

As the amount of epoxide necessary'is that amount necessary to provide an oxirane oxygen content in the fluid of at least 0.01% by weight. Generally from about 0.1 to 5.0% -by weight of the epoxide will be suflicient.

Examples of the polyhydroxy materials suitable for the use in the invention include sorbitol, mannitol, ethylene glycol, 1,2-propylene glycol, glycerol, pentaerythritol', erythritol, arabitol, dipentaerythritol, tripentaerythritol, etc. Thus, the polyhydroxy compounds have at least two hydroxy groups, two of the hydroxy groups being located no further apart than alpha or beta to each other; i.e., two of the hydroxy groups are separated by no more than 3 carbon atoms. The materials are aliphatic, since phenolic hydroxyls are not effective and the presence of aryl groups would make the attainment of a proper hydroxy to carbon ratio diflicult to obtain. Other functional groups may be present, as the carboxyl group in tartaric acid, halo, nitro, etc. The ratio of total carbon atoms to hydroxy groups will be from 1:1 to 2:1. A ratio in the range of from 1:1 to 15:1 is preferred. Thus, while 1,2-butylene glycol is effective, 1,2-pentylene glycol is not. The additional requirement of position of the hydroxy groups is likewise important. While 1,3-butanediol is effective, the hydroxy groups being in the beta position relative to each other, 1,4-butanediol is not effective. The preferred poly; hydroxy compounds are reduced sugars; sorbitol is especially preferred. 7

The amount of the polyol that is employed in the inhibitor combination is from about .01 to about 2%. The upper limit of the range is limited by solubility in the phosphate and usually very small quantities, from about 0.05% to 0.15% by weight, are sufficient and are preferred. I

In addition to the base phosphate fluids, the hydraulic fluids ordinarily contain a viscosity index improver. Ex-. amples of suitable high viscosity, indeximprovers include various acrylates and methacrylates. The preferred viscosity index improvers are polyalkyl acrylates. Also suitable and often used are polyalkyl methacrylates. In general, the materials are polymers having molecular weights within the range of from about 5,000 to 1,000,000. The prefererd materials have a molecular weight range of from about 10,000v to 250,000. The alkyl radicals of the polymerized acrylates and methacrylates will be fromabout C to C Examples ofemonomers from which the polyalkyl acrylates and methacrylates may be formed are acrylates and methacrylates having the following alkyl substituents: ethyl; n-propyl; isopropyl; n-, iso, sec-, and tertiary butyl; n-, iso, and tertiary amyl; hexyls such as n-hexyl, 2-ethy1-l-butyl, etc.; n-heptyl; 1,1,3,3-tetramethyl butyl; n'-octyl; 2'-ethylhexyl; and the various octyl, nonyl and decyl radicals, etc. Various combinations of the alkyl acrylates in the polymers are suitable. The viscosity index improver isemployed in amounts suflicient to raise theviscosity index to a point so that the fluid will possess adequate viscosity characteristics over the range of temperatures which are expected to be encountered during the operation-of the hydraulic system. Amounts of from 1 to 10% by weight are generally used. Thus, a fluid having a proper range will both lubricate the system and provide adequate power transfer over a wide temperature range.

In order to demonstrate the resistance to oxidation and corrosivity of the hydraulic fluids, two tests were employed, a small-scale oxidation test devised to permit screening of a large number of samples and Method 5308 from Federal Test Method Standard No. 791A.

In the small-scale test 7' ml. of the fluid were placed 0 in a 6-inch test tube immersed in an oil bath at about 260 F. Specimens of copper 0A" x /2" x and specimens of cadmium and magnesium /s" x /2 x & were placed in the tube. Air was blown through the mixture through a 1 mm. capillary tube resting on I the bottom of the test tube at a rate of about 2 bubbles per second. The appearance of the fluid and metal was rated on the following scale:

Rating Fluid Metal 1'. Unchanged Unchanged.

2 Yellow Light stain.

3 Light brown Dark stain or coating.

4. Dark brown Heavy coating or corroded.

Ratings of 1 or 2 were considered satisfactory, while ratings of 3 or 4 on either the fluid or any of the metals were taken as a failure point. The base fluid employed ,in the majority of the tests, designated dibutyl phenyl phosphate, is a mixture of materials containing, in addition (E) Estynox 308, glyceryl triepoxy acetoxy stearate, 3.6% oxirane oxygen;

(F) Estynox 408, isooctyl epoxy stearate, 4.5% oxirane oxygen;

(G) Epoxol 9-5, epoxidized linseed oil, oxirane oxygen 9% minimum;

(H) Epoxol 5-2E, epoxidized isooctyl tallate, oxirane oxygen 5% minimum;

(I) Epoxide 44, epoxidized fat;

(J) Epoxide 45, epoxidized fat;

(K) Glycidyl;

(L) 1,2-epoxy-3-alyloxypropane;

(M) 1,2-epoxy-3-(O-chlorophenoxy) propane;

(N) Glycidyl phenyl ether;

(O) Epoxol 7-4, epoxidized linseed oil], oxirane oxygen 7% minimum. 0

(P) 1,2-epoxydodecane;

(Q) Epoxycyclododecane;

(R) 1,2-epoxy-5,6-trans-9,IO-cis-cyclododecadiene;

(S) Epoxycyclohexane;

(T) 1,2-epoxyhexadecane;

(U) 1,2-epoxyoctoxypropane;

(V) 1,2,7,8-diepoxyoctane;

(W) Epoxyethyl-3,4-epoxycyclohexane;

(X) 3,4-epoxyvinylcyclohexane;

(Y) Pinene oxide;

(Z) 2,2-bis-(p-glycidoxyphenyl) propane.

EXAMPLE 1 The following table gives results obtained by the smallscale oxidation-corrosion test previously refererd to. The viscosity index improver employed and referred to as polyalkylacrylate was a mixed ethyl, 2-ethylhexyl, isodecyl polyacrylate having an average molecular weight of about 23,000 (number average). The 1-2-bis-phenyl thioethane employed in some examples for comparative purposes is a commonly employed oxidation-corrosion inhibitor. The epoxy compound was in each case TABLE I.-SMALL SCALE OXIDATION-CORROSION TEST Base stock Other additives Days to failure 1% glycerol 1% 1,2,6-hcptanetrioL 0.1% pentaerythritol. 1% diethylene glycol. 0.05% sorbitol. ..-1 .do

0.1% erythritol. 0.1% arabitol.-. 0.1% inositol 0.1% dipentaerythritol. 0.1% tripentaerythritol. 0.05?i sorbitol 0.5% BP'IE 5 7%ipolyalkylacrylate 0.5 BPTE 1. 0.5% BPTE+6.25% polyalkylacrylate- TABLE I.SMALL' soALn OXIDATION-CORROSION "rns'roonnnuea Base tbk 8 15 2950.) Polyhydroxy compound Other additives V I Days to failure Tributylphosphatc v x p Do 14 D 1 l r. i sh ltffifiiffiffitfitii: F 0 05% sorbitoi. v 14 1 Contains approximately 15% tributyl phosphate and butyl diphenyl phosphate. Z 1,2 -Bisphenylthioethane.

It may be noted that a large number of epoxide mate- It can be seen from these data that the inhibitor comrials of various types are effective in functioning as one bination is extremely effective; 1n reducing oxldatlon of of the inhibiting constituents of the invention. Thus, the fluid itself, as evidenced by the slgnlficant reduction in while the epoxides alone have a small inhibiting effect acid number increase over the.un1nh1b1ted materlal and in the tests, it may be seen that the polyhydroxy com- 15 also 1n reducing metal corrosion, especially of copper and pounds of the type previously described, when included in v cadmium. It may also be noted that the example cone the inhibitor combination, greatly increase the resistance talnlng BPTE, 1n combination wlth an epoxide, gave a to oxidation and corrosion. It may also be noted that 3.04 milligram per square centimeter weight-change of polyhydroxy materials in which'the hydroxy groups are the copper sample, indicating that it is decidedly less not so situated as in the described materials or the ratio of 20 effective than the polyols. Note that the epoxy compounds carbon 'to oxygen atoms is greater than 2:1 are not efcontaining only fused oxirane rings, Q, R, S, and Y, were fective in increasing resistance Th us, the example wherenot particularly effective in reducing metal weight loss in 1,2,6-heptanetriol was tried failed in 1 day. Also, the and one, plnene oxide, was exceptionally deleterlous to sample with neopentylglycol .failed in 2 days. In addition, cadmium and magnesium. L t t I the sample containing BPTE (1,2-bisphenyl thioethane), Also w1th1n the scope of the inventron is the combinaa known excellent oxidation-corrison inhibitor, gave an tion of the epoxlde and the polyol. The lnh bltor comll-day life in combination with Inhibitor A compared bination, thus, may be stored and transmitted in bulk with lives of greater than 14 to 18 days when A was comform for later addition to a phosphate ester base. The bined with various of the polyol inhibitors. combination will usually contain from 5 to 99.8%, prefer- 2 ably 80 to 99.8%, by weight of the epoxy compound. EXAMPLE In addition to the additives previously described, other In order to further demonstrate the eflectiveness of the additives may be included in the compositions. These ad- P -P Y Combinations of this invention, Samples ditives include supplementary viscosity index improvers, w tested y P y TeffiITed 10 Method pour point depressants, extreme pressure agents, and dyes, Reference to this method may be found in Federal Test Methods Standard No. 791A, Dec. 30, 1961. This method, We claim;

used for testing hydraulic oils to determine their ability to resist oxidation and tendency to corrode various metals, briefly consists of immersing five different metals in a single container of the specimen fluid at 250 F. for 168 40 hours while agitating the specimen with air and then 1. A hydraulic fluid consisting essentially of a major portion of a phosphate base fluid selected from the group consisting of trialkyl phosphates in which the alkyl groups contain from 1 to 30 carbon atoms, mixed alkyl aryl phosdetermining the acid number of the fluid and the weight pha'fes m Winch the aryl groups contain ifrom 1 to 3 loss measured in mg; per square centimeter of the various mat: nuclel and the alkyl groups contafn from to 30 metals which include aluminum Copper magnesium, carbon atoms, and mixtures thereof, a minor portion, not steel and cadimum. The base material employed in the test more than 5% y Weight, of was the same mixture employed in Example l-comprising all I Y compound having at least one n l l s d a major portion'of dibutylphenyl phosphate and minor oxirane ring selected from the group consisting of portions of butyldiphenyl phosphate and tributyl phosepoxi dized fats, ethers, and hydrocarbons. phate, said epoxy compound b'eing present in the fluid in amount Table II, following, includes data from this test. suflicient to provide an oxirane oxygen content of at least I Pmyhydroxy Metal wt. change mgJem Acid Base stock Epoxide compound Other additives Al Cu Mg Steel Cd number TABLE IL-FEDEREL METHOD 5308 OXIDATION END CORROSION 168 HOURS AT 250 F.

7 Metal wt. chan e cJn Polyhydroxy g mg s Base stock Epoxide compound Other additives Al Cu Mg Steel Cd nu et Dibutylphenyl phosphate t 21. 6 -3. 1 0 1. 1 Do 2 22. 22 0. 36 -0. 03 1.44 3. 54 Do. r A 0 22' 0.02 0. 01 0 06 0.58 Do A 0. 06 I 0. 05 0. 02 0. 05 O, 05 Do-.- A 0. 09 0. 09 0 0. 07 0. 02 Do F -0. 09 0. 02 -0-. 01 0. 07 0. 24 Do B 0. 12 0. 11 0.09 0. 11 0.01 D0 G O. 02 0 0 0. 02 l 0. 07 Do- H 0. 04 0. 02 0 0. 01 0.16 Do G --0. 1.3 0. 13 0. 06 --0. 09 v0. 07 D0 N 3. 04 0. 02 v 0 0. 02 0. 21 D 25. 10 O. 58 0. 00 -l. 10 3. 5 Do P 0. 08 0. 02 0. 01 --0. 03 0 04 Do. Q 4. 9 O. 60 O. 01 O. 37 2 26 Do R 13. 5 0.43 0.0 0. 9 3 22 Do--- S 10. 6 4. 7 0. 0 0. 9 2 Do T 0. 91 O. 36 O. 03 0. 17 0 30 Do U 0. 32 O. 04 0. 0 0. 14 0. 08 Do V 0. 02 -0. 01 O. 02 -0. 06 0 02 Do.- W O. 07 +0. 01 0. 02 --0. 15 0 03 Do- X 6. 1 +0.02 +0.02 0. l8 Do Y 19. 6 8. 6 O. 02 30. 0 5 1 Do t- Z 0. 22 0.00 0. 04 0. 05 0 08 0.01% by weight; and a minor portion, from 0.01 to 2% by weight of (B) an unsubstituted and carboxy substituted aliphatic polyhydroxy compound in which at least 2 hydroxy groups are separated by no more than 3 carbon atoms, and in which the ratio of total carbon atoms to hydroxy groups is from 1:1 to 2:1. 2. The fluid of claim 1 wherein (A) is an epoxidized fat.

3. The fluid of claim 2 in which (A) is an epoxidized drying oil.

4. The fluid of claim 3 in which (A) is epoxidized linseed oil.

5. The fluid of claim 1 in which (A) is an epoxy hydrocarbon.

6. The fluid of claim 5 in which the epoxy hydrocarbon has a boiling point above about 150 C.

7. The fluid of claim 5 in which the epoxy hydrocarbon is diepoxyoctane.

8. The fluid of claim 1 in the ratio of carbon atoms to hydroxy groups in (B) is from 1:1 to 1.5 :1.

9. The fluid of claim 8 in which the ratio of carbon atoms to hydroxy groups is 1:1.

10. The fluid of claim 1 in which (B) has hydroxy groups on three or more adjacent carbon atoms.

11. The fluid of claim 8 in which (B) is a reduced sugar.

12. An oxidation-corrosion inhibitor for phosphate esters consisting essentially of a mixture of from 5 to 99.8% by weight of an epoxy compound having at least one unfused oxirane ring selected from the group consisting of epoxidized fats, ethers, and hydrocarbons and from 95 to 0.2% by weight of an unsubstituted and carboxy substituted aliphatic polyhydroxy compound in which at least 2 hydroxy groups are separated by no more than 3 carbon atoms and in which the ratio of total carbon atoms to hydroxy groups is from 1:1 to 2:1.

13. The inhibitor of claim 12 in which the epoxy compound is an epoxidized fat.

14. The inhibitor of claim 13 in which the epoxy compound is epoxidised linseed oil.

15. The inhibitor of claim 12 in which polyhydroxy compound has a ratio of carbon atoms to hydroxy groups of from 1:1 to 1.511.

16. The inhibitor of claim 15 in which the polyhydroxy compound has a ratio of carbon atoms to hydroxy groups of 1:1.

References Cited UNITED STATES PATENTS 2,659,699 11/1953 George et a1 25278 2,862,886 12/1958 Davies et al. 25278 LEON D. ROSDOL, Primary Examiner P. E. WILLIS, Assistant Examiner US. Cl. X.R.

" UNITED STATES PATENT OFFICE /9 CERTIFICATE OF CORRECTION Patent No. 3, +87,02O Dated December 30, 1969 ln fl flfl) ROBERT L. PEELER and MANUEL A. PINO It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 5, after "9 4606" insert assignors to Chevron Research Company, San Francisco, California, a corporation of Delaware.-

The Assignment was recorded in the Patent Office on October 3, 1969, Reel 2537, Frame 350.

SIGNED AM.- SEALED JUL 2 1970 SEAIJ Attest:

| HE Edward M. Fletcher, Ir. WILLIAM Patents Commissioner Attesting Officer 

